The invention relates to a method of reducing the effects of varying environmental conditions, such as varying temperature or humidity, on the measuring results in a measuring instrument. The invention also relates to a measuring instrument having reduced effects of varying environmental conditions on the measuring results. The invention can be used with any type of measuring instruments the operation of which is dependent on ambient conditions. A preferred field of use of the invention is in connection with optical instruments, for example optical detectors for liquid chromatography.
Varying environmental conditions, such as variations of the ambient temperature or of humidity or of the speed of ambient air, have negative effects on the measuring results in a measuring instrument. In optical instruments, for example, temperature variations impair the measuring results in the following ways:
Thermal expansion of materials changes the dimensions of the optical components and may thus change the optical properties of the instrument Furthermore, mechanical stress is induced by thermal expansion which has an impact on optical image properties.
The emission characteristics of the light source of the instrument is temperature dependent.
The refractive index of a sample to be measured or of a solvent changes with temperature and influences the optical image properties.
The dark current and the sensitivity of a photo sensor used in the instrument is temperature dependent
The operation of the electronic circuitry used in the instrument for control and data analysis is also influenced by temperature.
In order to further illustrate the underlying problem of the present invention, reference is made to FIG. 1 which is a block diagram of an absorbance detector for a liquid chromatograph. The detector comprises a flow cell 1 with an inlet 3 and an outlet 2 for sample liquid eluting from a chromatographic separation column (not shown). The flow cell 1 is irradiated with a polychromatic light beam 4 generated by a tungsten lamp 8 and a deuterium lamp 6 and shaped by lenses 5 and 7. The light beam 4 entering the flow cell 1 is absorbed by the sample in the cell at specific wavelengths which are characteristic for the sample. The beam 9 leaving the flow cell 1 impinges on a diffraction grating 10 which spatially separates the beam 9 into rays of different wavelengths. The grating 10 directs the rays of different wavelengths onto a sensor 11, typically a photodiode array, where the rays of different wavelengths are detected. The absorbance detector shown in FIG. 1 further comprises electronic circuitry 12, e.g. for controlling the instrument and for data processing and evaluation, and a power supply 13.
The thermal balance and the distortion of the thermal equilibrium in the detector shown in FIG. 1 is determined by several factors, such as:
environmental conditions, mainly ambient temperature;
power dissipation of the tungsten lamp (typically 5 W);
power dissipation of the deuterium lamp (typically 25 to 30 W);
heat transfer of flow of heated sample or solvent (typically 0 to 20 W);
power dissipation of electronic circuitry, power supply and actuators.
The heat generated by the total power dissipation in the detector has to be transported to the outside. This can in principle be achieved by thermal conduction, thermal convection, thermal radiation, or forced air-cooling. In the prior art detector shown in FIG. 1, an arrangement 14 for forced air cooling, such as a fan, is employed for heat transfer. The arrows 15 indicate the direction of air flow. Air is thus transported by means of the fan 14 across the optical unit of the detector and then across the power supply and the electronic circuitry to the outside through openings in the detector housing.
The fan 14 also causes air to be drawn in through openings in the detector housing from the side opposite to the power supply and electronic circuitry (bottom of FIG. 1).
Under normal stationary conditions the power dissipation of electronic circuitry, lamps, actuators and power supply can be assumed to be constant and time invariant. The mentioned components are also only contributing to temperature effects on the measurement results to a smaller extent. Of greater concern are changes in the environmental conditions and the variation of the heat transfer of the sample or solvent as a consequence of a change of the analytical measuring parameters during the measurement.
Under normal operating conditions and with the assumption of constant ambient temperature the equilibrium is achieved after a certain amount of time, depending on the individual time constants of the various components. If, however, the ambient temperature is changing, all internal temperatures in the detector are changing according to the effective time constants given by thermal resistance and thermal capacitance. It can easily be shown, for example with the help of a model wherein the thermal conditions in the detector are expressed with electric circuit equivalents (heat as current, temperature as voltage, etc.), that ambient changes will modulate the internal temperatures. This causes thermal expansion, mechanical stress and therefore affects the optical properties of the system. The measuring signal is thus superimposed with temperature interaction effects so that the accuracy of the measuring results is impaired.
In the prior art, several attempts have been made to overcome the influence of ambient temperature changes. According to one approach, the forced air flow is varied by controlling the fan speed. In another approach, partial temperature control of specific functional blocks (optic or mechanic) is performed, for example by temperature stabilization of the optical unit, by using a flow heat exchanger, or by controlling the cooling air flow to the lamp housing. According to a third approach, it is attempted to keep the environmental conditions stable by employing air conditioning of the room in which the measuring instrument is located. In further prior art suggestions, sensitive functional blocks, for example the optical unit, are thermally insulated.
All the mentioned approaches provide only partial solutions to the problems associated with ambient temperature changes so that the effects of these changes on the measuring results cannot be suppressed or reduced to an acceptable level. Also, the prior art solutions are often technically complex and costly.
It is thus an object of the invention to provide a method of reducing the effects of varying environmental conditions on the measuring results in a measuring instrument which is comparatively simple to implement and which leads to a substantially reduced impairment of the measuring results by varying ambient temperature. It is also an object of the invention to provide a measuring instrument wherein the effects of varying ambient temperature on the measuring results are substantially reduced.
According to the invention, this object achieved by a method, with the measuring instrument comprising a measuring unit with components which are sensitive to varying environmental conditions, in that:
a) the measuring unit is thermally insulated such that the effects of variations in the environmental conditions on sensitive components are substantially reduced, but dissipated heat generated within the measuring unit can still leave the measuring unit; and
b) the temperature in the measuring unit is controlled by means of a control loop comprising a temperature sensor and means to influence the temperature in the measuring unit in such a way that the temperatures at locations with sensitive components are kept substantially constant.
For a measuring instrument, the above mentioned object is achieved in that,
a) a thermal insulation means is provided in the measuring instrument which substantially reduces the effects of variations in the environmental conditions on sensitive components, but still permits dissipated heat generated within the measuring unit to leave the measuring unit; and
b) control means are provided for controlling the temperature in the measuring unit, wherein the control means comprise a temperature sensor and means to influence the temperature in the measuring unit in such a way that the temperatures at locations with sensitive components are kept substantially constant.
According to the invention, it has been realized that one of the reasons for the shortcomings of the prior art solutions is that they are only designed to achieve thermal stability at a specific location in the measuring unit so that other locations are still influenced by external ambient temperature variations. The present invention, however, provides for a temperature control so that the temperatures at all locations within the measuring unit are kept constant.
In a preferred embodiment of the invention a heater and a fan are used to direct an air stream of controlled temperature to the measuring unit. The heater is controlled with the help of a controller which receives as an input the output signal of a temperature sensor measuring the temperature of the air stream.
In a further development of this preferred embodiment, the target temperature for the temperature control, i.e. the input of the controller, is dynamically adapted to the actual ambient temperature conditions by using an additional temperature sensor for sensing ambient temperature. Preferably, the control range is in the order of the expected variation of the ambient temperature and the target temperature is above the ambient temperature. This has the advantage that only a heating and no cooling is required in the temperature control loop and that the power consumption is comparatively small.
When using the dynamic adaptation of the target temperature to the actual ambient temperature short term fluctuations (in the order of seconds) and medium term fluctuations (in the order of minutes) of the temperature can be fully compensated and long term fluctuations would only be noticeable as a long term drift which, however, is uncritical in typical measuring instruments, for example in optical detectors for liquid chromatography, since such a uniform drifting can easily be taken into account when processing the measuring results. On the other hand, the dynamic adaptation ensures a low power consumption and a comparatively simple technical design at low cost